The German space center DLR is reporting that ROSAT — an astronomical satellite launched in 1990 — re-entered the Earth’s atmosphere just after 01:50 UTC last Sunday, October 23, burning up over the waters of the Bay of Bengal, to the southwest of Thailand. This was during the day, which is why there were no reports of it coming in, and no reports of debris hitting the ground. There are no reports of anyone getting hit, either, of course.

I was thinking about why it’s so hard to know exactly where and when a satellite comes down. New Scientist has a short, to-the-point article about it. In a nutshell, low-Earth orbit satellites like ROSAT are a few hundred kilometers above the Earth’s surface, circling every 1.5 hours or so. The Earth’s atmosphere doesn’t just stop if you go high enough; it thins out slowly, and even at 300 km there is still some air, though incredibly tenuous. However, over days, months, and years, a satellite moving through it feels a drag, feels air resistance. This drops its orbit, putting it into thicker air, which slows it faster, dropping it more… this process is very slow at first, but at some point increases rapidly, and the satellites drops into air thick enough that, at its high speed, causes it to burn up. I’ll note it’s mostly due to gas compression, not friction, that heats the satellite up.

And that’s why it’s so hard to know in advance where these things will come down. Solar wind and solar storms can puff the Earth’s upper atmosphere up even higher, increasing that drag. A heavy, small satellite has less drag than a light, big one. If the satellite is tumbling end over end, the drag changes. The interaction between the satellite and the air is incredibly difficult to model, and the best way to see what’s going on is to actually measure how fast it’s dropping and extrapolate. But even then, the exact place a satellite falls is hard to determine. They’re moving at 8 km/sec (5 miles/sec), so a few seconds off in the prediction means a huge uncertainty in where it will fall.

And, of course, the Earth is big. Very, very big. Chances are, there’s no one around to actually see where a satellite falls, so the exact location may never be known.

The funny thing, as New Scientist points out, is that the US has Earth-observing satellites that can probably see the infrared flare from a satellite as it burns up, and pinpoint the location. However, those are military (or NRO) satellites, so the data are secret — they’re used to look for the heat signature of missile launches. So, understandably, the government would prefer not to release the data, which would allow the bad guys to know our capabilities.

Over time, we’ll see more satellites coming down, and hopefully, like ROSAT, it’ll be over the vast waterscapes of the planet.

I’ll note it’s mostly due to gas compression, not friction, that heats the satellite up.

Actually, it’s due to passage of gas through a shock. This compresses the gas, but most of the heat is from dissipation of kinetic energy at the shock, not from compression. Compression (as in a bicycle pump) is a reversible process (one that does not produce entropy). Passage of gas through a shock, on the other hand, is inherently dissipative and causes an increase in entropy.

@1. Pete Jackson : “What do ROSAT and Osama Bin Laden now have in common?”

Burial at sea in the Indian ocean I presume?

@4. Chris : “But were any fish hit? No one ever cares about the fish.”

Unlikely any fish were hit unless they were especially unlucky flying fish!

Once ROSAT hit the ocean it’s momentum would, I’d expect, have been much slower and gentler as it sank and the fish would probably have been easily able to move out the way.

Jellyfish aside.

@7. Tara Li :

There’s enough of the bloody spy sats, …

For Your Information, ROSAT (short name for ROentgen SATellite) was an astronomical scientific research satellite not a spy satellite. It was a mission designed and run by the German Aerospace Centre to study the sky in X-rays, a space telescope. Click on my name for a link to their website.

…and the signature of a significant re-entry is going to be so different from a missile launch, they could post when & where with no real impact on security.

Please can you rephrase, expand upon or otherwise clarify that last part of your sentence there because I honestly can’t understand what you are trying to say or what point you are presumably trying to make. You are not making sense there – to me at least – at all.

I am not a native English speaker but I can very easily understand what Tara Li said. I’ll ty to re-phrase it for you, as you got her statemets completely wrong:

TL: “There’s enough of the bloody spy sats, …”

= “There are plenty of US owned military reconnaissance / early warning satellites in orbit that were able to trace the heat signature of the re-entering ROSAT…”

TL: “…and the signature of a significant re-entry is going to be so different from a missile launch, they could post when & where with no real impact on security.”

= “…so the USG knew withing almost real-time when and where ROSAT re-entered but refuses to let the rest of the world know as they are paranoid about disclosing their capabilities – which everybody already knows they have”

By the way, it’s officially the “German Aerospace Center”, not “Centre” I worked there for 10 years.

@11. George : Okay, that interpretation of what Tara Li was saying does make sense I guess when you put it like that – although I was kinda hoping (& still am) to hear from her directly on that. Thanks.

PS. Yeah, I always have been a lousy typer. Mea culpa. What’s “center” in deutsch anyhow?